Mixing panels wattages

Fast answer: Easiest to use panels identical or as close to what you currently have.
More info will be needed before an intelligent answer can be given.
Also, there are lots of panels a lot closer to 340 W. Higher wattage panels are most likely not more efficient, just physically bigger. Save a bit on wiring and pay for that with more considerations/potential problems you're probably not aware of.
Look around for same/close to what you have.

Thanks J.P.M.
I'll find something closer to 340W.
Out of curiosity, what would happen if I connected a 560W in series with a 340W?
Would I get 900W? or 680W.....or something else?
Do complications arise from the different voltages the panels have?
Thanks for your advice.

You're welcome.
Approximate answer: I don't know.

To repeat, more info would be needed before an intelligent answer could be given.

Mfg. of existing panels + model # ?
String inverter or micro inverters or optimizers ?

Kind of hard/scary to give safe advice about existing systems without being on site.
Too many little things that can cause problems that I (for sure) and probably you don't know about at this time.
Since it sounds like you're not in a DIY mode for doing what can be a moderately involved undertaking, IMO the best advice I can give: Learn about your system and then contact the installer.
Another short answer: Depending on the system particulars, mixing large differences in panel STC rating ratings in a system without some considerations isn't usually done without some modifications.
Stay with the same panels if possible. If not, stay as close as possible in terms of STC voltage, current and other electrical characteristics.

Once again....Many thanks for your help

Current matching is the most important part, not wattage. Using 60 and 72-cell panels in the same string is fine when they have the same Imp.

Thanks nerdralph,

The Imp/A for the Longi 565W is 12.96 (STC)
The Imp/A for the Seraphim 340W is 8.77 (STC)

Quite a difference.
Does this mean the impedance of the Seraphim panel will reduce the Longi panel Imp to 8.77?
Or do they add, potentially damaging the Seraphim panel?

Many thanks.

The currents would only sum if the panels were in parallel, not series. What will happen depends on the MPPT algorithm of your inverter. It could stop at around 9A, maximizing the output from the Seraphim panels, around 13A, maximizing the output of the LONGi panels, or somewhere in between.

The LONGi 565W panels have 182mm cells, which is the mainstream now. The older LR4-series panels have 166mm cells, with Imp around 10-11A. Before that, 158mm cells were common, with Imp around 9A. I suggest searching searching for old clearance inventory, even something with scratched or dented frames (but with no damaged glass).

Mixing different wattages of solar panels can be a bit like blending ingredients for a recipe - you want them to complement each other, not clash! While it's technically possible to mix panels with different wattages, it's essential to consider factors like voltage and current to ensure they play nice together in your system. Some solar companies might caution against it, fearing it could affect overall performance or efficiency. However, with careful planning and monitoring, it can work out just fine. Personally, I believe it's like building a mosaic; each piece contributes to the bigger picture, creating a unique and beautiful result.

Currently on my Commercial solar site I use First Solar 6 and 6+ panels. The 6's are 440W and the 6+ panels are 465W. How it was explained to me is I can stick a 6+ in a 6's spot and not notice a performance difference in the string of 6's. If I try to stick a 6 though into a string of 6+ panels, it will drop all the panels down to the performance of the 6. Now other factors are involved that prevent me from doing this such as the panels are slightly different sizes, but enough to screw up my racking system and require adjustments all the way down the line, so probably not a good idea unless it was a very end panel, and I was going from a 6 to a 6+ as otherwise my plants efficiency would be impacted. Once you get to the combiner box though it supposedly would not affect other strings going into the combiner boxes. Now all of this was learned through conversation with my panel manufacturer and a private homeowner might have trouble gaining that audience when they are talking 50 panels or less, but my site has over 600,000 of their panels installed and my company has contracted with them to supply panels to at least 3 other similar size sites giving me a little easier access to actual engineering team members.